This invention relates to in situ recovery of a resource from a deep subsurface formation. More specifically, this invention provides a method for recovering a resource from a deep subsurface formation by creating a horizontal plane of heated displacing means between boreholes in the lower portion of the subsurface formation, resulting in an extensive surface area for heat transfer into the upper portion of the formation.
There are many methods for recovery of a resource such as tar sands from beneath the earth's surface. Where there is little overburden, surface mining techniques have been employed. However when the overburden is thick or the ratio of overburden to tar sands thickness is high, then surface mining is not economical. Many in situ recovery methods have been proposed. For the deeper buried tar sands reservoirs, wells are drilled from the earth's surface down into the tar sand formation. A broad range of methods has been devised to establish both a communication path through the heavy, highly viscous bitumen-filled sand and an efficient method to recover the bitumen from the sand. These methods, such as fracturing, steam injection, fire flooding, solvent flooding, gas injection and various combinations of these operational steps, involve the introduction of steam, gas or other displacing fluid by means of vertical holes drilled into or in proximity with the resource formation. These processes generally involve the heating of the resource formation to reduce the viscosity of the resource, thereby allowing removal of the resource from the formation by hydraulic means or gravity flow. U.S. Pat. No. 4,160,481 uses a plurality of boreholes radially extending from a central shaft to inject steam into the resource formation. The steam is injected into some of the boreholes to drive the resource into the remaining borehouse where it is collected.
My invention, on the other hand, utilizes a horizontal heated plane of displacing means to greatly increase the exposure of the formation to the displacing means and thereby promote rapid and efficient transfer of heat to the resource. The horizontal heated plane is created by injecting heated displacing means into a plurality of boreholes within the resource formation. Unlike U.S. Pat. No. 4,160,481, heated displacing means is continuously added to the boreholes such that the resource in nearby boreholes is removed, thereby allowing the displacing means to laterally flow into nearby boreholes through the interstitial crevices between the boreholes vacated by the resource. The lateral flow of heated displacing means between the boreholes creates the most extensive surface area for heat transfer to the upper portion of the resource formation. The heated displacing means, such as steam, rises, condenses, and then drains, forming a local circulation cell. In addition, less heat is lost to the overburden since non-productive shales and sands above the tar sand will receive less heat from the process. When the heated zone reaches the height of the overburden, the process is nearly complete, and steam injection ceases in these boreholes thereby reducing the amount of heat transferred to the overburden.
In my invention, displacing means can be injected and removed from the boreholes simultaneously, thereby allowing the displacing means to be injected into all the boreholes at the same time if desired. By injecting displacing means into as many boreholes as possible, a larger horizontal heated plane is created, resulting in greater and more efficient heat transfer to the resource. Therefore, what is needed and what has been invented by us is a method for in situ recovery of a resource from a subsurface formation without the foregoing deficiencies associated with the prior art methods.